.TH std::filesystem::hash_value 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::filesystem::hash_value \- std::filesystem::hash_value

.SH Synopsis
   Defined in header <filesystem>
   std::size_t hash_value( const std::filesystem::path& p ) noexcept;  \fI(since C++17)\fP

.SH Parameters

   p - a std::filesystem::path object

.SH Return value

   A hash value such that if for two paths, p1 == p2 then hash_value(p1) ==
   hash_value(p2).

   The return value is consistent with std::hash.

.SH Notes

   Equality of two paths is determined by comparing each component separately, so, for
   example "a//b" equals "a/b" and has the same hash_value.

   hash_value originates from the Boost.filesystem library where it was used for
   interoperability with boost.hash (which calls hash_value found by argument-dependent
   lookup or boost::hash_value where available).

.SH Example


// Run this code

 #include <cassert>
 #include <cstddef>
 #include <filesystem>
 #include <iomanip>
 #include <iostream>
 #include <unordered_set>
 namespace fs = std::filesystem;

 void show_hash(fs::path const& p)
 {
     std::cout << std::hex << std::uppercase << std::setw(16)
               << fs::hash_value(p) << " : " << p << '\\n';
 }

 int main()
 {
     auto tmp1 = fs::path{"/tmp"};
     auto tmp2 = fs::path{"/tmp/../tmp"};
     assert(!(tmp1 == tmp2));
     assert(fs::equivalent(tmp1, tmp2));
     show_hash(tmp1);
     show_hash(tmp2);

     for (auto s : {"/a///b", "/a//b", "/a/c", "...", "..", ".", ""})
         show_hash(s);

     // A hash function object to work with unordered_* containers:
     struct PathHash
     {
         std::size_t operator()(fs::path const& p) const noexcept
         {
             return fs::hash_value(p);
         }
     };
     std::unordered_set<fs::path, PathHash> dirs{
         "/bin", "/bin", "/lib", "/lib", "/opt", "/opt", "/tmp", "/tmp/../tmp"};
     for (fs::path const& p : dirs)
         std::cout << p << ' ';
     std::cout << '\\n';
 }

.SH Possible output:

 6050C47ADB62DFE5 : "/tmp"
 62795A58B69AD90A : "/tmp/../tmp"
 FF302110C9991974 : "/a///b"
 FF302110C9991974 : "/a//b"
 FD6167277915D464 : "/a/c"
 C42040F82CD8B542 : "..."
 D2D30154E0B78BBC : ".."
 D18C722215ED0530 : "."
                0 : ""
 "/tmp/../tmp" "/opt" "/lib" "/tmp" "/bin"

.SH See also

                                    compares the lexical representations of two paths
   compare                          lexicographically
                                    \fI(public member function)\fP
   operator==
   operator!=
   operator<
   operator<=
   operator>
   operator>=
   operator<=>                      lexicographically compares two paths
   \fI(C++17)\fP                          \fI(function)\fP
   \fI(C++17)\fP\fI(until C++20)\fP
   \fI(C++17)\fP\fI(until C++20)\fP
   \fI(C++17)\fP\fI(until C++20)\fP
   \fI(C++17)\fP\fI(until C++20)\fP
   \fI(C++17)\fP\fI(until C++20)\fP
   (C++20)
   equivalent                       checks whether two paths refer to the same file
   \fI(C++17)\fP                          system object
                                    \fI(function)\fP
   hash                             hash function object
   \fI(C++11)\fP                          \fI(class template)\fP
   std::hash<std::filesystem::path> hash support for std::filesystem::path
   \fI(C++17)\fP                          \fI(class template specialization)\fP
